Speed control in a synchronous electronic motors with alternating monophase and polyphase current used in modern equipment and automation is effected by varying the frequency of the supplied current. This variation can be obtained by inverters which are capable of converting the network voltage having a stable frequency (usually 50 Hz or 60 Hz), into a voltage with a continuously variable frequency which can range for example, from one Hz to several hundred Hz.
Conventional inverters for this purpose used silicon controlled rectifiers (SCRS) which, however, have the problem that they are highly sensitive to transients in the network supply or generated in the inversion operation and which can cause destruction of fuses or overloads.
To avoid this problem, power transistors with analog modes of operation were substituted for the on/off switches constituted by SCRs.
More recently, inverters, especially in large scale integration (LSI), have been provided with MOSFET transistors or IGBT transistors and significantly differ from the previous SCRs or power transistors in that they are devices which control the voltage rather than the current as characterized prior power transistors and the like. The MOSFETS (Metal Oxide Semiconductor Filed Effect Transistors) and IGBTs (Insulated Gate Bipolar Transistors) are field effect transistors which have source, drain and gate terminals and which can be used to control significant power with a minimum gate power. These devices can be considered to consist of two diodes having a common cathode or anode.
Experience with MOSFETS, for example, in inverter circuits has shown that the MOSFETS are incapable of eliminating the problem of undesired switching due to transient parasitic voltages in the network with the danger that the electronic components will burn out, or the fuses will open circuit.
In order to supply a given load with a variable frequency f with the availability of only one direct current voltage generator V, it is sufficient to switch the flow of current in the load with the frequency f.
This commutation or switching must be performed with strictly constant time intervals so that the positive half wave and the main value of the alternating current circulating in the load has a value of zero. For this purpose, the inverter can be formed as a bridge circuit with at least two arms (two arms in the case of a single phase inverter and three arms in the case of a three phase inverter), each arm having two switches in respective halves of the arm of the bridge circuit. The output terminals are connected between the switches of the arms to the load and the arms are bridged between the direct current terminals of a rectifier network.
A simultaneous closing of the two switches in the same arm causes a short circuit of the power supply through the MOSFETs or IGBTs and/or the protective fuses or other components in the short circuit path.
When the MOSFETs or IGBTs are used, they constitute the switches in the aforementioned arms and the conventional inverter circuit will normally include a rectifier capacitor network producing the direct current output from the alternating current of the supply network, diodes for energy recovery and serving to protect the MOSFETs against destructive transient voltage surges, a control unit for sequencing the MOSFETs and any power supply required for the control unit.
It is important in the piloting of MOSFET transistors that the respective piloting circuits of the MOSFETs of a particular arm not have any common line as long as the drains of the MOSFETs are at different voltage levels.
One of the problems in dealing with inverters with MOSFET power transistors is false piloting signals due to transients which are invariably present in the supply network or generated in the load.
The transients occurring in other parts of the circuit, usually at capacitive components thereof, can be transmitted to the gate of the MOSFET setting it into conduction when it should be nonconductive, thereby detrimentally affecting the conducting pattern of the circuit. Generally, the source-drain diode component of the MOSFET does not interfere with the filing sequence since it is a fast diode.
To solve the problems with MOSFET invertors as described it has hitherto been required to provide complex pilot circuitry which frequently can cost 10 times the cost of the MOSFET to be controlled.